The invention relates to a printing group for a printing press, particularly a rotary offset press, having at least one plate cylinder, one blanket cylinder and one counter pressure cylinder, or a further blanket cylinder and a further plate cylinder.
A printing group of this type is known from European Patent document EP-A-0 531 880. In such printing groups, the elements involved in the printing process, such as gears, bearings and cylinders, must be produced and assembled with very high precision, because otherwise printing defects occur in the form of streaks, blotting or double printing. One such printing defect, known as channel streaking, appears more frequently as the print speed increases.
To avoid these defects, it is further known to mount so-called bearer rings on the two end faces of the plate and blanket cylinders. These rings are hardened, cylindrically ground steel rings that run onto one another under pressure. They reinforce the printing group by extensively eliminating the play and elasticity of the cylinder seatings, because the cylinders roll onto the bearer rings under a pre-stress. The bearer rings prevent the cylinders from decreasing their spacing in the cylinder channels, where the compressive stress temporarily breaks down, and then moving slightly further apart again with the next introduction of pressure.
The use of the aforementioned bearer rings, however, has the disadvantage that the spacing between the cooperating cylinders cannot be adjusted. During printing, the compressive stress, that is, the spacing between the blanket and counter pressure cylinders, should be adjustable to permit an adaptation to different paper thicknesses or paper surfaces. With cylindrical bearer rings, the thickness of the blanket cylinder cover would have to be changed for this type of spacing adaptation, which would in turn change the compressive stress between the blanket and plate cylinders. However, the compressive stress here should be constant. Thus, the cover thickness of the plate would also have to be changed. This in turn changes the printing length, which is not permissible in most cases.
Because of these problems, only bearer rings are used between plate and blanket cylinders in the aforementioned printing groups. The bearer rings extensively eliminate the channel impact between these two cylinders, but not the channel impact between the blanket and counter pressure cylinders or the first and a second blanket cylinders.
In European Patent document EP-A-0 531 880, the channel impact between the blanket and counter pressure cylinders or the first and second blanket cylinders is treated with special stops and cylinder arrangements.
It is also known to use rubber blankets in sleeve form to avoid the channel impact. Changing these blankets is an involved process, however, because, for example, the side wall and/or the respective cylinder seating on one side must be removed for changing the sleeves.